Biology (Greek, bios, life; logos, discourse) means the science of life. It treats of animals and plants. That branch of biology which treats of animals is called zoology (Gr. zoon, animal; logos, discourse). The biological science of botany (Gr. botane, plant or herb) treats of plants.

Living things are distinguished from the not living by a series of processes, or changes (feeding, growth, development, multiplication, etc.), which together constitute what is called life. These processes are called functions. Both plants and animals have certain parts called organs which have each a definite work, or function; hence animals and plants are said to be organized. For example, men and most other animals have a certain organ (the mouth) for taking in nourishment; another (the food tube), for its digestion.